Change in Vickers hardness at the glass transition region for fragile and strong glasses

The temperature dependence of Vickers hardness between room temperature and around the glass transition temperature Tg in a vacuum for some glasses with different fragile characters was measured. The hardness at T/Tg<0.9 decreases gradually with increasing temperature, but the glasses show unique temperature dependences in the range 0.9<T/Tg<1.1 depending on their fragile character. A sharp decrease in hardness is observed at around T/Tg=0.9–1.0 for fragile Bi2O3- and TeO2-based glasses, but 50CuOx·50P2O5 and soda-lime silicate glasses with strong viscosity characteristics still have relatively large Vickers hardness even at Tg and the hardness drops to almost zero around T/Tg=1.05–1.1. It is suggested that for fragile glasses the β-relaxation has an important effect on the Vickers hardness change near the glass transition region, but for strong glasses it is the α-relaxation that governs the hardness near Tg.